The efficient conversion of solar energy into chemical fuels has great economic and environmental significance. Visible light water splitting is a long-standing problem in photochemistry. Efficient photocatalytic water-splitting systems could have practical value for solar energy conversion, particularly if they could be coupled to higher temperature catalytic reactions for making liquid fuels. There remains a long-standing significant research interest in PEC water splitting using zinc oxide (ZnO)-based materials.
Hydrogen generation by photocatalytic water splitting using solar radiation is a renewable process, which can be carried out under ambient conditions for clean energy production. Several semiconductor oxides, sulfides and selenides have been explored which has led to various exciting and attractive developments in solar hydrogen producing system. However, attention is focused on oxide systems due to the availability, ease of synthesis and limited photocorrosion. Among these, zinc oxide has been garnering increasing interest due to its high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy. Moreover, ZnO is a chemically stable and environmentally friendly material.
Zinc oxide semiconductors are well known for their photocatalytic activity which originates from the utilization of photogenerated charges. When light of appropriate energy falls on a semiconductor, electrons are excited from valence band to the conduction band creating a hole in the latter. These charges can be effectively used in a variety of electron transfer or redox reactions provided that they do not recombine wasting the energy. In the case of a semiconductor bulk material, recombination sites are various, ranging from grain boundaries and bulk crystal defects to surface sites. In bigger particles, charges generated in the bulk encounter more number of recombination sites before reaching the surface for utilization in any reaction.
U.S. Pat. No. 7,338,590 discloses a method for generating hydrogen by photocatalytic decomposition of water using porphyrin nanotube composites. In some embodiments, both hydrogen and oxygen are generated by photocatalytic decomposition of water.
US patent application No. US20120145532 discloses a method to directly obtain clean hydrogen from solar radiation by using hybrid nanoparticles with metallic cores and semiconductor photocatalytic shells. Efficient unassisted overall photocatalytic splitting of water is based on resonant absorption from surface plasmon in metal core/semiconductor shell hybrid nanoparticles, which can extend the absorption spectra further towards the visible-near infrared range, thus dramatically increasing the solar energy conversion efficiency. When used in combination with scintillator nanoparticles, the hybrid photocatalytic nanoparticles can be used for conversion of nuclear energy into hydrogen.
Article titled “improved hydrogen production from water splitting using TiO2—ZnO mixed oxides photocatalysts” by A Pérez-Larios et al. published in Fuel, Volume 100, October 2012, Pages 139-143 reports TiO2—ZnO mixed oxides (1.0, 3.0, 5.0 and 10.0 wt. % Zn) photoconductors were prepared by the sol-gel method and used for the H2 production from water splitting. The solids were characterized by nitrogen physisorption, XRD, RAMAN, EDS, UV-Vis and XPS spectroscopy. High specific surface areas (85-159 m2/g) were obtained in all the mixed oxides compared to the bare TiO2 sample (64 m2/g). XRD and Raman spectra show that anatase is the predominant crystalline phase on the TiO2—ZnO solids. The band gap energy of the solids is in the interval from 3.05 to 3.12 eV which are in the same order than TiO2 (3.2 eV). These solids were proved in the photocatalytic water splitting and resulted six times more active (1300 μmol/h) than the reference TiO2 (190 μmol/h) semiconductor. This coupled TiO2—ZnO mixed oxides improves hydrogen production form water splitting.
Article titled “Hydrogen production from water splitting using perylene dye-sensitized Pt/TiO2 photocatalyst” by FS LIU et al. published in Acta Phys. Chim. Sin., 2007, 23 (12), pp 1899-1904 reports the photocatalyst (DPPBI/Pt/TiO2) was prepared using N,N′-di(4-pyridyl)-3,4,9,10-perylene tetracarboxylic acid bisimide (DPPBI) sensitized Pt/TiO2 and characterized by infrared spectroscopy (IR), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The results of characterization showed that the crystal form of TiO2 was anatase, Pt was highly dispersed on the surface of TiO2 and DPPBI was adsorbed on the surface of Pt/TiO2 in DPPBI/Pt/TiO2. Hydrogen production from water splitting using photocatalyst (DPPBI/Pt/TiO2) was studied.
Therefore, there is need in the art to develop the photocatalytic system which will minimised bulk recombination sites, spatial separation and increase the H2 production in water splitting. Accordingly, the inventors of the present invention developed photocatalytic composition which have the dual advantage of minimised bulk recombination sites using semiconductor nanoparticles and spatial separation and reduced surface sites using conducting organic linkers.